1. Field of the Invention
The present invention relates to a gravure printing plate and, more specifically, to a gravure printing plate capable of improving flow of ink at shadow portions thereof.
2. Description of the Related Art
Recently, sheet materials for flexible packaging formed of synthetic resin materials and the like are used for packing goods. Printing is carried out on the sheet materials for flexible packaging so as to provide good appearance and to indicate goods and contents thereof.
Gravure printing using a web-fed rotary gravure press, such as shown in FIG. 1, has been used for printing on the sheet materials for flexible packaging which is suitable for printing on the sheet materials.
Referring to FIG. 1, the gravure press comprises a gravure cylinder 7 on which a gravure printing plate 8 is wound, an impression cylinder 12 provided opposed to the gravure cylinder 7 for conveying the sheet material 11 for flexible packaging sandwiched therebetween, a doctor blade 10 for adjusting the amount of quick drying ink 5 supplied to the gravure printing plate 8, and a furnisher roll 6.
The web-fed rotary gravure press on the sheet material 11 for flexible packaging is generally referred to as "gravure for flexible packaging material", which is distinguished from "publication gravure" which is the web-fed gravure printing press on sheets of paper. The "gravure for flexible packaging material" includes three types dependent on the method of plate making, that is, conventional gravure, inverted halftone gravure, and electronic engraving gravure.
The gravure printing technique is briefly described in the following. An outline of gravure printing is disclosed in, for example, Japanese Patent Publication No. 63-28798. Referring to this document, in the gravure for flexible packaging material, a gravure printing plate is formed based on an original, and printing is carried out based thereon. The step of printing, i.e., the step in which ink is transferred from the plate to the material to be printed, is the most unstable step in which loss and deformation of characters become the largest, in the steps of transferring and reproducing image information from the original to the printed matter. The gravure printing plate is formed of minute concave portions called cells, having volumes corresponding to image density of the original.
As described above, there are three types dependent on the structure of the plate, that is, a conventional plate (FIG. 2A), an inverted halftone gravure plate (FIG. 2B) and an electronic engraving gravure plate (FIG. 2C). In any of the plates, the image density is divided into a light portion A, a middle portion B and a shadow portion C, in accordance with the volume of the cells. In printing, the ink 5 filled in the cells of the printing plate surface 8 is transferred to a material 11 to be printed. In the portion has relatively high image density, i.e., the shadow portion C where the cell volume is large, there is a flow of ink on the material 11 to be printed even in a section called a wall which is definitely distinguished on the plate surface 8. In the shadow portion C, the flow of ink 5 serves to assist the wall portion.
However, such a flow of ink causes a problem in accurately transferring image information which is formed in sections as cells on the plate surface 8. Especially in the inverted halftone gravure plate (FIG. 2B) or in the electronic engraving plate (FIG. 2C), the image information is reproduced based on the size of the opening of the cell, i.e., on the size of the dot on the material 11 to be printed, and accordingly, the quality of the printed matter is largely dependent on the change of the size of dots caused by the flow of ink.
The reason for this will be described with reference to FIGS. 3 and 4. FIG. 3 shows the relation between the density of an original and the density of a printed matter in gravure printing. FIG. 4 shows, in enlargement, the relation between the plate surface F and the print surface P in a light portion A', middle portions B' and B" and a shadow portion C' of FIG. 3.
In principle, the ideal relation between the densities of the original and the printed matter is as shown by the line I of FIG. 3. However, in practice, the above-described flow of ink is generated at the region in the middle portion, for example between the portions B' and B", where the density of the original changes slightly. Consequently, in the portion B", adjacent dots are brought into contact with each other so as to generate an abrupt change of the density of the printed matter between the portions B' and B". Namely, the density of the printed matter becomes as shown by the line II, in which a jump of density is generated at a certain portion of the middle portion, which causes reproduction with uneven tone. The portion where such jump occurs changes dependent on various conditions of the press machine (speed of printing, angle of inclination of the doctor blade, pressure of the impression cylinder, hardness, and so on), the plate (depths of cells, forms of cells and so on) and of the ink (composition, viscosity and other characteristics).
In gravure printing, the dots of the printed matter are unavoidably brought into contact with adjacent dots at the middle portion of a certain density, as shown in FIGS. 3 and 4. The reason for this is that portions formed of smaller cells are more susceptible to the influence of ink flow compared with the portions formed of larger cells or portions formed of rough point patterns formed of groups of smaller cells, even if the surface area per unit occupied by the cell portion on the plate surface is the same.
Examples of the above described gravure printing plates 8 comprise those disclosed in Japanese Patent Laying-Open No. 59-232347 (hereinafter referred to as a conventional example 1) and those which are commercially available shown in FIG. 6 (hereinafter referred to as a conventional example 2).
FIGS. 5 and 6 are both partial enlarged perspective views schematically showing plate surfaces of the gravure printing plates.
In the conventional example 1 shown in FIG. 5, the cells C for filling ink are formed by walls 2, and in the shadow portion, intersecting portions of the walls 2 are removed to form unwalled portions 3, with the bottom surfaces of the cells C continued through the unwalled portion 3.
Meanwhile, in the conventional example shown in FIG. 6, the unwalled portions 3 are formed near the intersecting portions of the walls 2 in the shadow portion and the unwalled portions 3 are formed to be arranged in a horizontally or vertically staggered manner. The bottom surfaces of the cells C are continued through the unwalled portions 3.
FIG. 7 shows a gravure printing plate 8 having the form of the conventional example 2 shown in FIG. 6, viewed from above. As shown in the figure, the ink 5 flows in a prescribed direction through unwalled portions 3. The ink 5 flows through a prescribed direction on the surface of the plate, since the ink flows to a direction opposite to the rotation of the rotary press, as the printing plate 8 is rotated in a prescribed direction on the rotary press, as shown in FIG. 1. In gravure printing, since the flow of ink is approximately in a prescribed direction as the process of printing proceeds, and the ink is of a quick-drying type, uneven flow of ink like an orange peel is generated. Those skilled in the art call this uneven flow of ink "mottling". Mottling is especially conspicuous in the shadow portions (e.g., C in FIG. 2A).
Therefore, conventionally, generation of "mottling" is reduced by setting the above mentioned conditions of the printing press and of the printing plate 8 at prescribed values based on experience. However, it is very difficult to maintain and control operating conditions. It is difficult in both conventional examples 1 and 2 to eliminate "mottling", since the unwalled portions 3 continue with prescribed pitches. When "mottling" is generated, the surface of the printed matter will appear to have thin stripes.
The details of the "mottling" will be described in the following. As shown in FIG. 1, after the ink 5 is transferred from the cells of the printing plate 8 to the object 11 to be printed, it is fed to a drying box 13 where the ink is dried. The state of the ink transferred from the cells of the printing plate 8 to the object 11 to be printed is shown in FIG. 8A. Referring to FIG. 8A, when the ink 5 is transferred onto the object 11 to be printed, the ink of each cell has a convex shape. Although there is a small flow of ink between the convexes of the ink after the ink 5 is transferred until the printed matter 11 is transferred to the drying box 13, the ink 5 does not become completely flat as shown in FIG. 8B, since the ink is of a quick-drying type and the flow of ink is in only a prescribed direction. Consequently, the heights of the convexes of the ink become varied, causing uneven density of ink. The unevenness of the ink density depends on the viscosity of the ink (viscosity), color, and the depths of the cells. In addition, it depends on the speed of printing (fast, slow). The unevenness of ink is generated when the printing speed is low and when the speed is high. When the speed is high, the stripe-shaped unevenness becomes more conspicuous.
In addition, the unevenness of ink depends on the screen ruling (175, 150l, 100l, and so on). Namely, if the ruling is small, i.e., if the mesh is rough, the unevenness becomes more conspicuous.
In the conventional example 1, all intersecting portions of the walls 2 are removed. Therefore, the wall portions 2 are cut into small pieces, the walls 2 wear much during printing, and the life of the walls 2 is not very long.
Although the problem of short life of the conventional example 1 is solved and the above-mentioned "mottling" can be reduced at a specified speed of printing in the conventional example 2, "mottling" is still generated when the speed of printing is changed.